ThreadPoolExecutor: 基本任务处理流程

Author: c-rainstorm

java/ThreadPoolExecutor源码剖析.md

@@ -147,6 +147,49 @@
 
 ## 任务处理流程
 
+我们以一个简单例子来剖析一下整个过程。
+
+```java
+
+package me.rainstorm;
+
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.LinkedBlockingDeque;
+import java.util.concurrent.ThreadPoolExecutor;
+import java.util.concurrent.TimeUnit;
+
+/**
+ *
+ * @author baochen.zhang
+ * @date 2017.12.4
+ */
+public class ThreadPoolExecutorDemo {
+    private static final int THREAD_POOL_SIZE = Runtime.getRuntime().availableProcessors() + 2;
+    private static ExecutorService exe = new ThreadPoolExecutor(THREAD_POOL_SIZE, THREAD_POOL_SIZE,
+            60, TimeUnit.SECONDS, new LinkedBlockingDeque<>(100));
+
+    {
+        ((ThreadPoolExecutor)exe).allowCoreThreadTimeOut(true);
+    }
+
+    public static void main(String[] args) {
+        exe.execute(() -> System.out.println("Hello world"));
+
+        exe.shutdown();
+        exe = null;
+    }
+}
+
+```
+
+![](../res/thread-pool-executor-demo.png)
+
+1. `ctl` 变量之前已经提到了，作为一个控制变量用来控制线程池的状态和工作线程数。默认值是 `RUNNING | 0`，即状态为 `RUNNING`，Worker 线程数为 0;
+1. 在 Demo 中并未指定线程工厂，`ThreadPoolExecutor` 使用 `Executors` 提供的默认线程工厂。
+1. 因为只有一个任务，所以 `main` 方法中在提交完这个任务后，直接调用了 `shutdown()` 方法，并将其赋为 `null` 便于在任务执行完毕后回收资源，一般情况下推荐在所有任务都提交到线程池以后再调用 `shutdown`，否则之后的任务直接会被拒绝掉。
+1. 因为只有一个任务，且允许核心线程超时，所以该线程在 `getTask()` 过程中会超时，然后返回 `null`，进入 `processWorkerExit()` 流程。
+1. 线程池在进入 `TERMINATED` 状态后就可以被 GC 了。
+
 ## 最佳实践
 
 一般情况下使用 `Executors` 的工厂方法来创建即可适用于大多数场景。需要配置的话参考 个性定制 来配置更合适自己项目的 `ThreadPoolExecutor`。
@@ -161,3 +204,4 @@
 1. [Java SE 9 & JDK 9 -- Source Code](.)
 1. [What is Daemon thread in Java?](https://stackoverflow.com/questions/2213340/what-is-daemon-thread-in-java)
 1. [深入理解java线程池—ThreadPoolExecutor](http://www.jianshu.com/p/ade771d2c9c0)
+1. [Java 中, 为什么一个对象的实例方法在执行完成之前其对象可以被 GC 回收?](https://www.zhihu.com/question/51244545)

res/thread-pool-executor-demo.png

@@ -0,0 +1,77 @@
+@startuml thread-pool-executor-demo
+
+participant ThreadPoolExecutorDemo
+participant ThreadPoolExecutor
+participant Worker
+participant Thread
+participant Executors
+participant DefaultThreadFactory
+
+ThreadPoolExecutorDemo -> ThreadPoolExecutor : new ThreadPoolExecutor();
+activate ThreadPoolExecutor
+    ThreadPoolExecutor -> ThreadPoolExecutor : ctl = new AtomicInteger(ctlOf(RUNNING, 0));
+    activate ThreadPoolExecutor
+    deactivate ThreadPoolExecutor
+    ThreadPoolExecutor -> Executors : Executors.defaultThreadFactory()
+    ThreadPoolExecutor <-- DefaultThreadFactory : java.util.concurrent.Executors.DefaultThreadFactory;
+    ThreadPoolExecutorDemo <-- ThreadPoolExecutor : ThreadPoolExecutor instance
+deactivate ThreadPoolExecutor
+
+...
+
+ThreadPoolExecutorDemo -> ThreadPoolExecutor : allowCoreThreadTimeOut(true)
+
+...
+
+ThreadPoolExecutorDemo -> ThreadPoolExecutor: execute(command);
+activate ThreadPoolExecutor
+    ThreadPoolExecutor -> ThreadPoolExecutor : addWorker(command, true)
+    activate ThreadPoolExecutor
+        ThreadPoolExecutor -> Worker: new Worker(command);
+        activate Worker
+            Worker -> DefaultThreadFactory : newThread(this);
+            Worker <-- DefaultThreadFactory : Thread instance
+            ThreadPoolExecutor <-- Worker : Worker instance
+        deactivate Worker
+        ThreadPoolExecutor -> ThreadPoolExecutor : workers.add(worker)
+        ThreadPoolExecutor -> Thread : Thread start()
+    deactivate ThreadPoolExecutor
+deactivate ThreadPoolExecutor
+
+ThreadPoolExecutorDemo -> ThreadPoolExecutor : shutdown()
+activate ThreadPoolExecutor
+    ThreadPoolExecutor -> ThreadPoolExecutor : advanceRunState(SHUTDOWN);
+deactivate ThreadPoolExecutor
+
+... worker thread start ...
+
+Thread -> Worker : run()
+activate Worker
+    Worker -> ThreadPoolExecutor : runWorker(this);
+    activate ThreadPoolExecutor
+        ThreadPoolExecutor -> ThreadPoolExecutor : beforeExecute(wt, task);
+        ThreadPoolExecutor -> command : run()
+        activate command
+            note over command
+                需要执行的任务，当前任务是
+                System.out.println("Hello world");
+            end note
+        deactivate command
+        ThreadPoolExecutor -> ThreadPoolExecutor : afterExecute(task, thrown);
+        ThreadPoolExecutor -> ThreadPoolExecutor : getTask()
+        activate ThreadPoolExecutor
+            ThreadPoolExecutor -> ThreadPoolExecutor : compareAndDecrementWorkerCount()
+        deactivate ThreadPoolExecutor
+        ThreadPoolExecutor -> ThreadPoolExecutor : processWorkerExit()
+        activate ThreadPoolExecutor
+            ThreadPoolExecutor -> ThreadPoolExecutor : tryTerminate()
+            activate ThreadPoolExecutor
+                ThreadPoolExecutor -> ThreadPoolExecutor : ctl.compareAndSet(c, ctlOf(TIDYING, 0))
+                ThreadPoolExecutor -> ThreadPoolExecutor : terminated()
+                ThreadPoolExecutor -> ThreadPoolExecutor : ctl.set(ctlOf(TERMINATED, 0));
+            deactivate ThreadPoolExecutor
+        deactivate ThreadPoolExecutor
+    deactivate ThreadPoolExecutor
+deactivate Worker
+
+@enduml